texts: What is the characteristic time of the evaporation of the galaxies?

texts:
For a star, to leave its galaxy, it requires probably a 3-body near-collision at the edge of the galaxy. It is unlikely, but possible. And, if a star once somehow got the required velocity to escape the galaxy, then it will escape and never comes back.

Thus, galaxies should have a characteristic time of their evaporation, what could be calculated. I suspect, this time is probably longer, than other related processes (expansion of the Universe, end of the age of the stars, etc), but it exists can it can be probably calculated (most likely, by numeric simulations).

It probably also depends on the size and star density of the galaxy.

Was it calculated? How big is it?


texts: What mechanism causes oscillations of the solar system's orbit about the galactic plane?

texts:
In a recent paper (news release here) Lisa Randall and Matthew Reece propose that a dark matter disk coinciding with the galactic plane together with the solar system's oscillations through the galactic plane could explain the 35 million year periodicity in mass extinctions.  They propose that the solar system passes through this dark matter disk in the galactic plane every 35 million years, disrupting bodies in the Oort cloud and causing some of them to collide with Earth.  An image of the proposed cycles from the news release is below.

My question is: What mechanism is responsible for this 35 million year oscillation about the galactic plane?  Is there a companion body to the sun? Are we orbiting around one of the arms of our galaxy?  Is this a well known phenomenon or are they proposing the 35 million year oscillation as well as the dark matter disk?


texts: How Would a Neutron Star Actually Appear?

texts:
Having seen many pictures produced by artists of neutron stars and planets that orbit some of them, I was wondering how a pulsar would appear to a human being, in visible light (assuming the intense radiation etc. doesn't kill us in the process).

As I understand, the pulsar's beam is projected from the star's magnetic poles rather than rotational poles, which are not necessarily in line with each other. Given that pulsars rotate extremely quickly and the beam could be visible across vast distances - such as if it were shining through the pulsar's nebula - would it appear as a straight line, curved line or perhaps a cone? This is assuming the beam can be seen in visible light.

Given the incredible density of neutron stars and their small physical sizes, would the night sky be visibly distorted to the point where (for example) just after sunset on a hypothetical planet, one could possibly observe other planets near or behind the star that would otherwise be blocked by it?

Given their small surface areas, would a neutron star still appear as luminous as say, the Sun, at a similar distance? How close would you have to get to a neutron star for its apparent magnitude to match the Sun's from Earth?


texts: Can earth escape sun's gravity with the help of a black hole heading towards our solar system?

texts:
To escape the Sun's gravity, the Earth would need sufficient amount of energy. Let's say a stellar black hole has entered our solar system and at a certain distance from earth, the black hole exerts same and equal gravitational force on the Earth as the sun is exerting. Earth feels a zero net force. Will it help earth to fly away?

Assuming the black hole had an empty stomach before eating our solar system.


texts: Calculate latitude and longitude based on date and sun

texts:
I can't seem to find this algorithm, if it exists. Can you calculate the longitude and latitude of where you are standing based on the date and the angle of the sun? Also, would the algorithm work for any date past or future?

Thanks for any help, AD


texts: How can the observable universe be so small if there are so many stars in it?

texts:
The observable universe has a radius of about 46.5 billion light years. That’s big but I just wonder how that can be big enough to fit in everything we know exists in the universe. There are hundreds of billions of galaxies and each galaxy has billions of stars.

Plus there are vast spaces between galaxies. And galaxies tend to cluster up into local groups, so there would be even bigger gulfs of empty space between these local groups.

If the observable universe was only 46.8 billion light years, and there were this many and galaxies stars that need to fit inside, wouldn’t everything be much closer together?


texts: How did the authors of Surya Siddhanta find the diameters of other planets in the solar system?

texts:
The Surya Siddhanta, "a Sanskrit treatise in Indian astronomy from the late 4th-century or early 5th-century CE" is truly a great work.

But how was it possible for the writers to find the exact values of the diameters of different planets and the distance between the sun and the earth?

Also, are there any mistakes in this book?

If there's any problem in my question please inform me.


texts: How much mass per year may the black hole at the center of the Milky Way gain by capturing neutrinos?

texts:
If we are immersed in a sea of neutrino, then a black hole should be constantly capturing the ones crossing its event horizon, and gaining mass.

How much neutrino mass falls inside a black hole per year? For example, the one at the center of our galaxy.


texts: we know that our universe is expanding, but we must agree to the fact that it has a limit. It just cant expand till infinity. Eventually there will be a end to time. I thought alot about this and i have a idea that before big bang there was nothing except time and a room like structure in which the big bang happened and the walls of the room are the limits of the universe. It does not mean all there was before big bang a room, but it was the room in which our universe came to existence. After The Big Crunch i believe the room will contain nothing it just like before big bang. Now I want to know that does this idea of mine have any logic?


texts:
A galaxy cluster consists of somewhere around a few tens to several hundred galaxies.

To measure the position of a galaxy cluster, i.e. it's RA and DEC, do astronomers always assume it to be its Brightest Cluster Galaxy BCG's position?

It has been shown by Johnston et al. 2007 that BCGs are not always found in the centre of a cluster/halo. If so, does the assumption of taking the position of a BCG to be the position of the galaxy cluster still hold true?


texts: Bounded Observable Universe and General Relativity

texts:
The book General Relativity for Mathematicians by Sachs and Wu defines a "spacetime" as a connected 4-dimensional, oriented and time oriented Lorentzian manifold.  I emphasize the word "connected."  There are those that say the observable universe is bounded, because the universe is expanding faster than the speed of light.  A bounded observable universe that is isolated from the rest of the universe, strikes me as a disconnected manifold, and would not seem to comport with general relativity's notion of spacetime.  Is this correct?

Following up on the above point, wouldn't there have to be some sort of discontinuity, at the moment when the universe splits off into isolated pieces?  How many discontinuities are being hypothesized?  Does general relativity contemplate discontinuities in spacetime and gravity?


texts: How to explain tidal tails and dark matter sub-halos in simple terms?

texts:
There is the exciting news by ESA  entitled Is the nearest star cluster to the Sun being destroyed?

Data from ESA’s Gaia star mapping satellite have revealed tantalising evidence that the nearest star cluster to the Sun is being disrupted by the gravitational influence of a massive but unseen structure in our galaxy.

I heard about dark-matter sub-halos and tidal tails before, but I am by no means an expert and Wikipedia is not too helpful for my question: I would like to use ESA's animation on Hyades tidal tails to explain the terms "dark matter halo" and "tidal tails" in 2 minutes. Consider that my audience probably heard the term "dark matter" before, but they have to be reminded about it. The term "galaxy" may be used, maybe even "star cluster", but not much more. May you help me, please?
References

Tereza Jerabkova et al.: The 800 pc long tidal tails of the Hyades star cluster


texts: How was the distance from the Earth to the Sun determined

texts: How was the Earth to sun distance measured. How do we know how far from the Earth the Sun actually is.


texts:
I've been listening to an episode of the "NASA in Silicon Valley" podcast about the SOFIA mission.

One of the techniques they use at SOFIA is stellar occultations to study different objects.

My impression was that the occultations by Pluto happen much more often than by Triton. Is this true and why is that?


texts: How do astronomers know when the Sun will die?

texts: How it is possible for astronomers to find out when the end of the Sun's life will be?


texts: Need help with a simple Blackhole question

texts: Suppose the amount of mass in a black hole doubles. Does the event horizon change? If so, how much does it change?


texts: Why was Neptune rather than Uranus chosen as an archetype?

texts:
The largest exoplanets are typically called Jupiter-like planets, or hot/cold Jupiters. Since Jupiter is the largest most massive planet in our Solar system, it makes sense that it would be chosen as an analog. For intermediate-size exoplanets, Neptune is used as an analog (e.g. 1).
Neptune and Uranus are both roughly the same size and mass. So why is Neptune rather than Uranus used as the archetype of intermediate-mass planets?
Did someone call an exoplanet a Neptune-like planet, choosing Neptune over Uranus for no particular reason, and then everyone else followed suit? Or is there a fundamental difference between the two planets that makes Neptune a better analog?


texts: What did LIGO Actually See? (Gravitational waves discovery)

texts: I am trying to find an original video/image of what LIGO actually saw, but all I can find is artist renditions of gravitational waves.


texts:
For many years the existence of medium sized Black Holes (IMBH) have eluded scientists. BH of several times the mass of our sun have been found, as well as SMBH with millions of sun masses. SMBH's and small ones mass grows with time, as matter gets transferred to the accretion disc and later gets absorbed or by merging. Quoting a paper from 2018:

Although many IMBH candidates have been identified, none are accepted as definitive; thus, their very existence is still debated.

My question is: If we know BH grow, where are the medium sized Black Holes? How come small sized ones are easier to spot than medium ones? Why is their mere existence beign debated on?


texts: How does the sun move over the year in the sky of the Moon?

texts: Wikipedia's section on the Moon's inclination says it is always 1.5° to the ecliptic. Does that mean that as seen from its surface, over the year the sun at zenith would move 1.5° to the north, then to the south, varying by 3° overall?


texts: Bousso bound within particle horizon, discrete Q.field theory

texts:
I have 2 questions:

Could you please give me arxiv numbers of papers dealing with the Bousso bound for the particle horizon (known Universe), from masters level upwards, with also some papers a bit more philosophical or vulgarised?
It has been said that due to Bousso bound (or Bekenstein's, I don't remember), QFT or any other field theory should have a limit in the micro dimensions, therefore be discrete instead of continuous: do you know some arxiv papers with the same levels and approaches than in (1)?


texts: Conversion of Magnitude to flux using zero point in AB Magnitude

texts:
I want to calculate the flux of the 5 sigma limited magnitude. It is given that the zero point magnitude of NB964 is 28.77 and 5sigma magnitude is 25.45.

The following example is from A large, deep 3 deg2 survey of Hα, [OIII], and [OII] emitters from LAGER: constraining luminosity functions

  …file to measure the auto magnitude per source. The measured zeropoints are 28.77 and 32.37 mag for the NB964 and DECam z band, respectively, and all source magnitudes further mentioned in this paper are based on 2” apertures. The 5 $\sigma$ limiting AB magnitudes are 25.45 and 25.84 mag for NB964 and DECam z, respectively.

(original screen shot)

What I am trying to calculate is to convert it to flux by using this equation

$$\text{flux} = 10^{-(48.6 + \ \text{mag} - \ \text{zero_mag})/2.5}$$

But I am not getting the correct answer. I got 7.727e-19.

They mentioned that the 5sigma corresponding flux is 8.2e-18.

  …of our survey. The $5 \sigma$ narrowband magnitude limit is 25.45 mag, corresponding to a limit line flux of $8.2 \times 10^{-18}$ erg s-1 cm-2.

(original screen shot)

How to get the correct flux?


texts: Are there any planets or moons denser than Earth?

texts: Earth has the highest density out of all planets, planetoids and moons of our planetary system, and also has a higher density than the Sun. Do we know any exoplanets or moons denser than Earth?


texts: I have a filterbank data (say name.fil) which I have dedispersed and stored into name.dat file using PRESTO. May I ask for detailed steps and explanation on how to convert that data to sound?


texts: The sun "burns" hydrogen and even has "campfires" on it, but has anyone calculated a rate of actual chemical burning on the Sun?

texts:
Answers to Hydrogen burning vs Hydrogen fusing explain that in astrophysics "burning" generally refers to nuclear fusion or at least nuclear reactions1, and information at

Why didn't we see those campfires on the Sun until now?
What could be the cause of solar "campfires"?

explains that those are astronomical nicknames for very small bright bits of plasma recently discovered on the Sun, not actual fires.
But in the Sun's atmosphere there may (or may not) be opportunities for chemical reactions. Certainly in some cool stars the atmosphere even makes dust.
Question: The sun "burns" hydrogen and even has "campfires" on it, but has anyone calculated a rate of some actual chemical burning on the Sun? Any oxidation reactions at all? If not our Sun, is oxidation simulated for some other stellar atmospheres?

1it's hard for me to think of two protons "fusing into a diproton", later beta-decaying for example; feels more like a nuclear reaction, but maybe that's just me.


texts: Would the electron cyclotron-maser emission mechanism affect Proxima b's ability to retain an atmosphere?

texts:
In a recent arXiv preprint, Pérez-Torres et al. "Monitoring the radio emission of Proxima Centauri" claim the detection of radio emissions synchronised with the orbit of the planet Proxima b. They explain this as the result of electron cyclotron-maser (ECM) emissions, making the Proxima–Proxima b system a scaled-up analogue of the Jupiter–Io or Jupiter–Ganymede systems.
Would this mechanism make it more difficult for Proxima b to retain an atmosphere?


texts: Dealing with damp and dew whilst stargazing at night or in the early morning

texts:
Sometimes when using my telescope late at night or early in the morning, especially in the more humid of seasons, I notice i sometimes get dew buildup on the lenses.

How can I minimize damp buildup whilst using my telescope?

Simply dabbing away the liquid with a dry cloth doesn't seem to do the trick as I am still left with some water tracks and residue that I just can't soak up, this then means that all of the light is refracted providing a very fuzzy and blurry image.

What preventative measures can i take to stop damp build up in the first place?


texts: Collision of asteroid and gaseous planet - what would happen

texts:
What happens when asteroids (or other small bodies) "collide" with a gas planet? In my head it would either go through it or just stay "inside" it due to the strong gravity. I tend to believe in the latter one, but if this is the case, does this mean that the gas giants are full of small bodies inside them?

Thank you.


texts: M87 Black hole. Why can we see the blackness?

texts:
So, as the title states, why are we able to actually see the 'blackness' of the black hole? I get that what we are actually seeing is the event horizon, or accretion disk. But should this not extend all the way round? Surely the black hole is not a 2D thing so we can "look into it from above" (I use that term loosely as obviously there are no directions in space!), so why are we able to actually see the blackness?

The only thing I can think of is that a black hole, much like our solar system has some sort of ecliptic, that the vast majority of matter is orbiting, and everywhere else just doesn't have enough matter for the light to be visible, kind of like why we are unable to see the Oort cloud.

I hope that made sense, and I could be way off, but it is the only thing I could think of to explain it. If it is the case, then would we be able to get a similar image of Sagittarius A, seeing as we could be in this "ecliptic" of it, so surely we would only be able to see the heated matter around the event horizon and not the blackness?


texts: Why is the Warm-Hot Intergalactic Medium (WHIM) so hot, and what is "collisionless shock heating"?

texts:
The Phys.org article Researchers find last of universe's missing ordinary matter says:

Ordinary matter, or "baryons," make up all physical objects in existence, from stars to the cores of black holes. But until now, astrophysicists had only been able to locate about two-thirds of the matter that theorists predict was created by the Big Bang.
In the new research, an international team pinned down the missing third, finding it in the space between galaxies. That lost matter exists as filaments of oxygen gas at temperatures of around 1 million degrees Celsius, said CU Boulder's Michael Shull, a co-author of the study.

and refers to F. Nicastro et al, Observations of the missing baryons in the warm–hot intergalactic medium, Nature (2018). DOI: 10.1038/s41586-018-0204-1
The Wikipedia page Warm–hot intergalactic medium says:

Part of the gravitational energy supplied by these effects is converted into thermal emissions of the matter by collisionless shock heating

but the link to "collisionless" is ambiguous and I haven't a clue how a shock wave can get hot without collisions.
Is there a way to understand why is the Warm-Hot Intergalactic Medium (WHIM) is so hot, and what the "collisionless shock heating" process is that appears to be heating it?
Are the temperatures associated with the motion of the atoms and with the excited states of the atoms similar? With only of order 1 atom per cubic meter, collisions must be so infrequent that it seems to me that it might not be so hard for these two temperatures to diverge.


texts: What is the overlap between the Gaia and the Hipparcos catalog?

texts: Is there any kind of concordance available between the sources in the Hipparcos catalog and the Gaia DR2 catalog? I.e., mapping the Gaia DR2 source_id to the Hipparcos HIP number?


texts: Venus-Saturn conjunction in Sagittarius on Dec 11, 2019 - How often does this happen?

texts: Venus and Saturn will be approximately two finger widths apart in the constellation Sagittarius on December 11, 2019. I was wondering: how often do Venus and Saturn come together in the sky like this? I would assume it wouldn't be that frequent. Is it quite frequent? Is there a formula to predict something like this?


texts: How does gravity have an effect from the inside the event horizon of a black hole with the rest of the universe?

texts:
According to general relativity, as I understand it, the space around a black hole's event horizon is distorted by gravity, such that the distance to a point approaching the event horizon from an observational point further out from the EH approaches infinity.

So if the distance to any mass inside the EH appears to be infinite, how does this mass assert gravitational effects outside the EH?

Perhaps a different question, what is the difference between the gravitational effects of a mass inside the EH of a black hole and a similar mass that is outside the cosmic event horizon of an observational point?

(edit as follows)
My original intent for this question is rethink how we think about black holes.  The distance to a point on the event horizon of Sag A is as far to the point on an observer's cosmic event horizon.  You can travel no farther than such a point on the CEH than you could to a point on Sag A's event horizon, at least using our 4D coordinate system.  BTW, I think all talk about a black hole having a singularity at its center is bunk, first of all, we can never get there to confirm or deny. Secondly, the density of Sag A if it were uniform, would mathematically would be around the density of liquid water as it exists on earth, but we would never be able to know, just as we would we would ever be able to know the density of the universe outside of our cosmic event horizons.  Any answers to those two questions would be meaningless anyway, as long as there is no away around the speed of causality rule enforced by our universe.

Now I believe I have read that Hawking Radiation has been shown to exist.  However, does that mean the part of whatever is outside our cosmic event horizon is evaporating as well?


texts: How to attach binoculars on starwatcher tracker?

texts:
I am a complete newbie on trying to figure out how to attach the binocular on the starwatcher tracker.
I am having problem with the angle. This is the issue I am having:

I align the skywatcher tracker to the north star (Polaris), (I am at 40 degree longitude)

I attach the L shaped bracket on the skywatcher tracker. This L shaped will be able to support my binocular.  Once I attach the binocular which it sits parallel on the short leg of the "L" shaped bracket i.e. parallelly aligned to the pole star.

Now the problem is as as follows: I have the polar axis. Also consider a plane which is perpendicular to the polar axis passing through my center of my tripod. I am able to view objects on the side of the plane which has the north star. But I am unable to view any objects on the opposite side. If I turn my binocular around it will have towards the ground instead of the sky.

Am I missing an attachment here, or do I have something wrongly configured? Any help or comments is much appreciated.
Thanks in advance.

photo added. Say the binocular is focused at the north star. But the object I am trying to watch is behind the plane which is defined by the green L-bracket in the photograph. the binocular can rotate about the polar axis (which is perpendicular the plane defined by the green L-bracket)

I have two degrees of freedom without disrupting the polar alignment. I can rotate along the x or y axis. If I try to rotate anything using the parts in the circle I will loose polar alignment.
In order to observe something right of the plane (defined by: on which the y axis lies that is the y-z plane): First I will have to rotate the binocular along the y-axis by 180 (but that makes it point to the ground). Second I will have to rotate along a z-axis to point it to the sky -- which I have no controls to do.


texts: Could there be a planet at the barycenter between two or more stars revolving around each other?

texts:
In binary star systems, could there be a planet the stars revolve around, with eternal day on all sides?
1st scenario: Imagine a binary system consisting of two Sunlike G5V stars of 1 solar mass each, orbiting each other, and at the barycenter between them is a planet (which would make the planet itself revolve at the same horizontal speed around its axis, making each sun appear always above the same location on the planet). If the stars revolved around the planet both sides would be illuminated similarly. I don't see why it should be impossible. A planet that once was the outermost one in orbit around one of the stars got ejected from its orbit by the other star's gravity, migrating into the barycenter between them.
2nd scenario: Imagine there's a planet at the Lagrangian point between Alpha Centauri A and B. If the stars revolved around the planet both sides would be illuminated similarly. Would that be possible?
Do situations like those above occur or was such even observed?


texts: Focal ratio importance for planetary viewing

texts:
It seems that almost every article online regarding planetary viewing recommends a telescope with a high focal ratio (f/8 or more), for revealing the finer details of the planets. But I find this a bit confusing because the resolving power of a telescope is solely a function of the aperture, or at least that is my understanding.
Given the above, shouldn't it be a matter of a large enough aperture and then the eyepiece to use to zoom on the detail of choice in formed image?


texts: Sinusoidal Light curve of an eclipsing binary

texts: This light curve seems really weird to me, I'm used to seeing light curves where the barycenter is a straight line. It either stays at a place, moves at a constant speed or slowly increases its speed. But here it looks like the barycenter periodically changes its speed. Why is this happening? Can someone please explain the details of this graph.


texts: Convert Object Intensity to Pixel Intensity

texts:
If I have an object of some size and spectral radiance, and it's some distance R from a camera, how would I calculate its digital count value in an image? I'm sure there's an equation out there, but I don't know what to search for. It would be a function of integration time, aperture, focal length, gain, quantum efficiency, point-spread function, spatial resolution, etc. For simplicity, I'd assume the noise is Gaussian, and that atmospheric transmission is perfect.
Can someone please help?


texts: Have we discovered any natural satellites of natural satellites of planets or dwarf planets? Even very small, or relatively short-lived - e.g. ringlets around Saturn's moons, some meteorites orbiting Jupiter moons, or something to orbit Charon? Or is the Star-Planets-Moons the deepest naturally ocurring orbital recursion level?


texts: Could Tabby's star be beamed by a neutron star's jet causing irregular star spots?

texts:
There have been some other questions here about what might cause the light curve dips of star KIC 8462852 as observed by Kepler Space Telescope. Here's one I haven't heard before:

Could it be that a neutron star, from Earth yet unobserved, or another supernova remnant in the relative vicinity of Tabby's star, hits it with its polar jet stream? Being in the focus of an irregular jet of charged particles would disturb the star's magnetic field, somehow causing star spot patterns, and upsetting its photosphere perhaps otherwise too.

Can this be excluded?


texts: I measured the FWHM (Full Width at Half Maximum) of several emission lines using SPLOT in IRAF, and now need to calculate the uncertainty. How can I do that?


texts: Information Paradox with Hawking's Radiation

texts:
A theory about Blackholes storing the information from the universe.

Being said that Blackholes are condensed mass sufficient enough to break every particle into different elementary particles, broken particles will eventually reach the center(or they break down the time they reach there).

Some say that the information is stored on the surface instead, which can be carried by the Hawking's Radiation.

Questions:

How exactly can Hawking's Radiation carry the information?
How is it possible for information to exist at the event horizon?

(If the mass was swallowed by the blackholes then every single particle should be inside the event horizon as they brought towards the black hole at very high velocity and it's becoming difficult to imagine that some particles were left at the horizon itself).

Maybe: If the information is on the event Horizon then it might not have broken to elementary particles!


texts: Why does Mars appear to retreat across the sky?

texts:
If you plot the movement of Mars across the Earth's sky it appears to move back on itself during its orbit.

Why does it appear to have a loop shape from Earth?


texts: Complex life in binary black hole - Sun(s) system

texts:
I was wondering if there could theoretically exist systems in which an Earth-like planet was in the habitable zone of one (or two) Sun-like star(s), while actually not orbiting around any star.
To be more precise, I started asking myself whether a planet like earth could be placed in the inner orbit of a black hole, while one or a pair of Suns would be revolving around the black hole too, on an outer orbit.

My question is not whether such a system could actually exist somewhere, but only if it could theoretically exist, even if it is probabilistically not likely to exist.

So I found this sets of issues:

Planet must be far enough from the black hole ( > Roche Limit )
Harmful radiations from black hole's accretion disk
Planet-Star relative distance must not vary too much...which I guess in turn means that the planet-blackhole distance must be small compared to the average blackhole-star distance

Given these limitations, how would you choose the stars and blackhole masses and their relative distances to the planet, in order to have it suitable for complex, perhaps intelligent forms of life? Do you see other issues to this purpose?

Below I add what I've been able to collect so far (more like a collection of unsolved questions..)

So, about the issue #1, I thought about a rather lightweight blackhole, with M < 10 Solar masses. I once read in a thread that when considering a central body = 10 solar masses, and a satellite with a density of 5000 kg/m^3 (Earth's average density), then the Roche Limit would be as little as 0.8% of an AU. That would be conveniently small, though I guessed that using Earth's average density may not guarantee that the whole planet is kept together (it may lose its atmosphere...). So, just to be sure, would you rather increase this distance to 1/20 of an AU, perhaps less, perhaps more? I'm asking here because I'm not able to conduct these calculation myself.

Then again, I'm not an expert and I don't know if staying out of the Roche Limit is a sufficient condition for orbiting around a blackhole of this small size without things becoming nasty. I know of issue #2 though,and about that I read someone suggesting that If the planet were subjected to tidal lock, then at least the face looking away from the blackhole would be spared from the harming X rays. But how close a planet should get to a blackhole in order to experience a tidal lock within a reasonable amount of time? Would it likely be tidally locked if it stayed at 0.05 AU away from a 10 solar masses black hole? I guess the answer to be yes, but I wanted some validations from the experts here.

Then about issue #3 again, I thought that If there were two Suns (making another binary system of their own), then there would be also a larger insulating flux, perhaps allowing us to keep the Suns more far away (by a quick calculation I think I found that two Suns at 1.4 AU would give the same flux as one Sun at 1 AU). Another possibility is to have a brighter Star class, at expense of its life-time. So for simplicity let's say we can go up to 2 AU (maybe even more) without the Planet getting too cold, or the Stars too short-lived.

Summarizing, I now have a 10 Solar Masses black hole and a (likely) tidally locked earth-like planet orbiting at 0.05 AU from it. Tidal forces should still be small enough not to suck the planet's atmosphere. Perhaps having a planet slightly bigger than the Earth would also help in this account. Then we have a couple of Stars orbiting together around the black hole, at a distance of 2 AU. Relative distance from the twin Stars to the Planet is 2 +/- 0.05 AU. I guess that would be within the limits of the habitable zone, right?

One last curiosity: assuming the black hole has an "average" spin, would it be realistic if the two Stars had a tilted orbit with respect to the black hole spinning axis?

I'm sorry for the many questions, by any means please don't assume I'm expecting a single person to answer all of them.


texts: What would the Sun be like if nuclear reactions could not proceed via quantum tunneling?

texts: Without quantum tunneling our Sun wouldn't be hot or massive enough to produce the energy it does at the moment. So what would have been the temperature or mass of our Sun without quantum tunneling of protons to maintain the same energy we receive from our Sun?


texts: Will a planet's core always be very hot?

texts: Including formed & forming planets, will the core of a planet always be very hot, or are there any planets with cold cores?


texts: Why is molecular hydrogen (H2) so difficult for astronomers to detect?

texts:
I am reading a great deal about the various forms of hydrogen throughout the universe, and I keep reading that 'molecular hydrogen is notoriously difficult to detect', and other sentiments along those lines...
Why, exactly, is molecular hydrogen gas so difficult to detect?


texts: What is the farthest point of light that is emitted by a torch?

texts: If I focus my torch into the night sky,what would be the farthest point the light can reach? Recently I looked at the night sky and noticed a light spark moving around the clouds for about 15 min.After observation I jumped into a conclusion that its probably because of the light house near by or any powerful light emitter.


texts: I'm 8 hours from Greenwich, why is my sidereal time 10 minutes different from theirs?

texts:
In order to write and run the script in this answer
I typed "Greenwich sidereal time right now?" into google (or at least tried to) and got two numbers.
Greenwich sidereal time was 09:51:07 but my local sidereal time was ten minutes earlier at 09:40:50.
I'm in UTC +8 so the ten minute difference is not because I'm ~8 hours away.
What causes this small difference in sidereal times?


texts:
Why can't we use the semi-minor axis in Kepler's third law? The formula for the third law is:
$$\dfrac{a^3}{T^2} = \text{const.} $$
Instead of semi-major axis (a) why can't we use the semi-minor axis (b)?
$$\dfrac{b^3}{T^2} \neq \text{const.} $$
I just checked it (I calculated the results for some planets using the semi-minor axis) and I really don't know why it's not constant.


texts: How did we first determine the Earth's departure from sphericity?

texts: I'm quite interested to know the history of the study of the Earth's departure from spherical symmetry. What were the first methods used to model the non-spherical geometry of the Earth? For example, there are geodesy satellite and missions used now to determine the gravity field of the Earth but that is "recent" technology. They obviously had other methods prior to this.


texts: Can radioactive decay rates be confirmed by looking at supernova?

texts:
Radiometric dating relies on accurate knowledge of the half-lives of radioactive elements. Skeptics have argued that since many elements' half-lives are too long for anyone to have observed them, we can't know what they were in the distant past.

There are various counterarguments based on terrestrial evidence, like agreement between elements in the same sample, and corroboration from varves, tree rings, and ice layers.

Another counterargument I've seen is by looking at supernova, we get a window "back in time" to when their light began its long journey to us, and that much of this light was generated by radioactive decay, confirming that decay rates were the same then as now.

I don't quite follow this explanation, and would like some help. Distant starlight doesn't give us a rock sample in which to analyze proportions of parent and daughter elements. Does it still somehow give evidence of specific radioactive decay rates? If so, how?

I know next to nothing about astronomy, so please explain this to me like I'm a child.


texts: Are there large underground caverns on Mars?

texts:
Scientists talk about the possibility of life underground on Mars, and I assume they are talking about microscopic life that can live in small gaps in the soil or rock.  But is there any evidence that there might be large caverns under the surface of Mars relative to the human scale or even larger?

Then I wonder how could we even detect or map such subsurface features on Mars or other rocky bodies in the solar system such as Titan, Venus, or even the Moon.  Could we use radar or maybe sonar for bodies with substantial atmospheres from an orbiting spacecraft?  Are there other methods for detecting sub-surface cavities?


texts: When was the last volcanic eruption on Mars?

texts:
There are several volcanoes on Mars, including Olympus Mons, the largest volcano in the solar system, the volcanoes on the Tharsis bulge,... But they are all thought to be inactive now.
When did the last volcano erupt on Mars?


texts: Is the material that could have formed a "Super-earth" divided among the four terrestrial planets

texts: I know "super-earths" are quite common in other planetary systems, but there are none in the Solar System. Is it due to the material that could have created a single large super-earth being used to create four smaller terrestrial planets in the Solar System?


texts: What was that single file line formation of flashing plane lights flying in the sky at night?

texts: It was here on the west coast at 5 am pacific standard time when I went outside for a smoke when out of the corner of my eye what looks like a plane with flashing lights going across the sky when all of a sudden there was another... and another... and another! For more than 15 minutes I watched these "planes" flying overhead in a single file line formation. There's no way that many planes or satellites traveling in a single file line formation from the east, over the pacific ocean, to the west and heading inland. I've NEVER seen anything like that before in my almost 30 years of living and stargazing. WHAT DID I SEE?!?! I have to know because this was by no means a normal evening/morning. To top it off there were other "planes" or "satellites" flying in the same direction as the others traveling in the single file line formation almost as if they were off course and ended up flying outside of the designated flight pattern. Nothing I've looked up explains this... Why? The date is april 12th 2020 and even though it's a pretty old phone I got a 3 and a half minute video before I went back inside to look them up on the internet only to find NOTHING. The sun is peaking and starting to hide the stars and I CAN STILL SEE the single file line formation of flashing plane lights in the sky traveling overhead more than 30 minutes later. What on gods green earth did I witness?


texts: Why do we use solar filters instead of simply reducing aperture of the telescope?

texts:
Everywhere I read that to view the Sun in the telescope, one has to use a solar filter. The filter must be handled carefully to avoid any holes in it and even mustn't be stretched so as to avoid reducing its reflectivity in some spots. Otherwise it becomes dangerous due to possibility of overexposing/burning observer's retina.

But, if all we need is to reduce illuminance, and covering part of the telescope's aperture won't obscure the image (since the aperture is far from being in focus), why not simply reduce the aperture to a very small hole? Photo cameras do this all the time, why wouldn't this work for telescopes? Or does a solar filter block IR/UV light, in addition to reducing visible illuminance?


texts: Number of lenslets in wavefront sensor array

texts:
I have to do a short calculation, but, quite frankly, I have no idea how to even start...
Suppose you have a 10-m telescope (f/10) with an infrared camera that observes at 10 micrometer. The seeing is 2 arcsec (FWHM of the PSF,  2”= 1/100000 rad) and you would like to implement an adaptive optics system.
The question is: How many lenslets do you need in the Shack-Hartmann wavefront sensor array? Below is a picture of the working principle of such an array. The telescope would also include a coronagraph, but I think (might be wrong) you don't need that information for this calculation.


texts: Astronomical data convertion from Jy/pixel to MJy/sr?

texts:
I'm in the middle of collecting infrared data of various wavelengths. My problem is the following: SPIRE data (250µm) are in MJy/sr and PACS data (100µm) are in Jy/pixel. I would like to combine these data, but I'm a bit lost with these units... So I would have several questions:

How can we switch from Jy/pixel to MJy/sr?
Is "Jy/beam" equivalent to "Jy/deg^2" or "Jy/sr"?


texts:
The ALMA (Atacama Large Millimetre/submillimetre Array) radio telescope's band-10 capabaility is now operational, per this answer. That's confirmed by NRAO's First Science with ALMA’s Highest-Frequency Capabilities.

According to this site ALMA's bands and frequencies are as follows:

band  wavelength   noise   frequency
        (mm)        (K)      (GHz)
 1    6.0 - 8.5     26     35 -  50
 2    3.3 - 4.5     47     65 -  90
 3    2.6 - 3.6     60     84 - 116
 4    1.8 - 2.4     82    125 - 163
 5    1.4 - 1.8    105    163 - 211
 6    1.1 - 1.4    136    211 - 275
 7    0.8 - 1.1    219    275 - 373
 8    0.6 - 0.8    292    385 - 500
 9    0.4 - 0.5    261    602 - 720
10    0.3 - 0.4    344    787 - 950

900 GHz (0.9 THz) is quite a high frequency for a radio receiver! Each ALMA dish down-converts received frequency to a baseband of a few GHz before they get digitized and sent to the correlator for digital interferometry, but you still need an ultra-stable local oscillator (LO) for downconversion, and all of the LO's of all of the dishes need to be mutually coherent. That's quite a feat considering they can be tens of kilometers apart!

Question: How does ALMA produce stable, mutually coherent ~THz local oscillators for all of their dishes?

As suggested in this partial answer to the question How does the Event Horizon Telescope implement the interferometry? hard drives collect digital data down-converted by local atomic clocks, probably using GPS as a secondary reference, then brought to a central location for post analysis.

I am guessing that they spend a significant amount of time trying to reconstruct coherence at the millimetre (picosecond) level, but that option is not available for ALMA as data is continuously recorded at some large fraction of 24/7, and the huge volume of data at a location where magnetic hard drives don't work is overwhelming. So they need to get it right the first time.


texts: Once a black hole is formed, is there anything other than Hawking radiation which shortens its life?

texts:
Hawking radiation is supposed to very slowly evaporate a black hole (terms and conditions apply :] ).

Apart from Hawking radiation, is there any mechanism or effect that can make a black hole cease to exist? Or once they are formed are they expected to exist in this form "forever"?

PS: I posted the same question to Physics but I think it applies well to both SEs. So I hope it is okay to try here as wwell.


texts: What is the distance that the Moon travels during one orbit around the Earth?

texts: Also, does it always take the same amount of time, or does it fractionally differ on each revolution?


texts:
I'm new to astronomy and have to make a star map that displays the stars above a current location. From the information I've pieced together I understand that declination is equal to the current latitude of the observer, however I am slightly confused about right ascension. My two quesions are;

If I work out GMST using the equations here; http://www2.arnes.si/~gljsentvid10/sidereal.htm will the answer be the current right ascension above Greenwich or are there more steps I've missed?
If I want to find the sidereal time of, for example, Sydney, Australia, do I need to account for daylight savings at either location or do I simply add 11 hours to GMST?

Thank you for your help.


texts: Since the Universe is expanding, is it accurate to say that a galaxy is 5 billion light years away?

texts:
Isn't it better to say that the light from the galaxy took 5 billion light years to reach us and it is a very old picture of it? However, if light travelled back to the galaxy at the constant speed and an observer was stationed to record it wouldn't it take longer to reach because of the expansion of the Universe?

Also if it still exists after 10 billion years or not we could not predict. So is the picture of the Universe more of a timeline of events rather than like a conventional map that shows distance between points.

How do current day theories tackle these issues?


texts: Why don't multiple eclipses happen over the course of a few days?

texts:
From my understanding over the course of the seasons the moon and sun rise at varying locations. Though the day to day differences are no noticeable, to me at least. For instance, in the summer, the sun takes a longer path overhead as a result of the earths tilt towards the sun in the summer months -- in the winter, short paths.

With the seemingly negligible difference in path lengths of the sun from day to day why then do solar eclipses only happen on a single day and not over the course of many days? It seems that if the moon and sun paths during the day were to intersect they should do it multiple days in a row as their path lengths across the sky during the day change.

I assume the answer to this lies particularly with the moons orbit around the earth and its shape/inclination. But I am far from my last astronomy courses and have lost all the jargon and knowledge needed to determine this.


texts: Can a planet in our system eclipse the sun as seen from another one?

texts:
When the Sun, Earth and Moon get properly aligned, we get eclipses, where the Sun is partially or totally hidden by the Moon when seen from Earth.
Is it possible for one planet of the solar system to produce an eclipse on another one?
There are two underlying questions in this:

are alignments of the Sun and two planets actually possible (and if so, how frequent)? Given the slightly different orbital planes of the planets, this seems difficult?

even though planets are much smaller than the Sun, with appropriate distances between the three bodies considered, is it actually possible for one planet to partially or completely hide the Sun like we see in the Sun-Moon-Earth case, or does it only happen for planet-satellite combinations given the relative distances (i.e. in all other cases if would just be a tiny point transiting across the Sun)?

I understand that the Earth would not be able to project a shadow far enough to get such a phenomenon on other planets, but are there other combinations where it’s possible/did happen?


texts: What are the equations governing stellar evolution (Luminosity, Mass, Temperature, Radius)

texts:
I'm looking for ('simplified') equations governing stellar evolution. Especially how mass, luminosity, temperature and radius of a star change during it's lifetime. As well as equations which tell you how long a star stays in a certain stellar stage (PMS, MS, ...).

Extra clarification by example:

...

After that a star with a mass between x and y enters the main sequence for a duration of $\Delta T = formula$. During this phase it's mass changes according to $M = formula(t)$. after that it enters the xxx  phase

...

For those who think this question is to broad, please give some suggestions, 'google terms', ... from which I can move forward.


texts: Term for a momentary geometric pattern formed by astronomical objects

texts:
Few days back, the Pleiades, the Sun and the Moon were forming an almost perfect equilateral triangle.

Is there a term that describes such momentary geometrical patterns? Asterism and conjunction are close, but not exactly applicable here.


texts: Is there a maximum temperature a planet can have?

texts:
This may sound a little bit confusing, but I will try to explain. (I have done research on this topic already)
I have looked at the temperatures of planets and I was wondering, is there a point where a planet is at perigee and it's temperature cannot reach any higher? Let's just say the planet is tidally locked, and is currently at perigee. Would the planet be at a point where it's temperature can no longer be increased at the current distance from its star?


texts: A beginner project in plotting celestial bodies in 3D coordinate space

texts:
As a data visualization task, I'd like to plot some celestial bodies in 3D coordinate space. I assumed there would be a lot of databases where celestial bodies are recorded along with an XYZ coordinate relative to our sun. As I ponder more on it I begin to realize other features, for example how is the axis of alignment achieved and agreed upon?

Given the below data set, is it clear what system is in use and how I may interpret it in a way that gives me an XYZ I can begin plotting?

https://www.google.com/fusiontables/DataSource?docid=1TZ3eoWstpR8d0O2-eqoz3VFDsLwSSzNA5wX4LxE


texts: There are exoplanets called Gliese 581c, Gliese 581d etc. I assume their star is Gliese 581. So where does this Gliese come from?


texts: Aligning the equinoxes to the cardinal points on a circular calendar

texts:
I want to draw a calendar as a circle, with the equinoxes at the top and bottom, and the solstices at the left and right. I am prepared to accept a little inaccuracy by having exactly 365 or 366 days in the circle depending on if it is a leap year.
If I just divide the circle into 365 equal segments, then it does not line up. I can get the solstices to line up very closely, but the spring equinox always comes in 2 days late, and the autumnal equinox 2 days early.
I think this is to do with the elliptical orbit of the Earth around the Sun, and the relationship of the axes of the ellipse to the axial tilt of the Earth.
Can anyone help me with the mathematics required to adjust the angle taken up by each day in order to skew the days such that the equinoxes and solstices fit to the cardinal points of a circle?


texts: How does the illumination of the sky from the sun compare to that of the moon?

texts:
I have trouble describing the questions but this is the best I can do.
The sun lights up the sky, until it's about 18 degrees below the horizon. The moon also lights up the sky, and i'm wondering, how does the illumination of a full moon compare to that of the sun? How many degrees does the sun have to be below the horizon to equal the illumination of a full moon?

I hope i'm clear enough. Thanks for any answers


texts: Beneficial astronomical uses of satellite constellations like Starlink?

texts:
Has any thought been given to software that would use predictable occultations by Earth-orbiting satellites to improve (rather than degrade) astronomical observations?

Potential challenges I could imagine might include:

The satellites’ high angular velocity compared with typical occulters.
Knowing enough about the outline and orientation of each satellite.
Diffraction effects.

Perhaps there are satellite design changes that could make them more useful, akin to star shades. Perhaps some camera or other sensor design changes could help capitalize on what seems an inevitable reality. And perhaps there are types of observations for which no amount of cleverness and computational resources could turn Starlink into a net win for astronomy.


texts: superimpose borders of several FITS on the sky map

texts:
I have several FITS files with correct WCS info (as far as I know) and want to project them or preferably their borders only in the same sky projection, so it will be easy to understand an actual intersection of different files.
A good illustration of what I want is http://astro.dur.ac.uk/Cosmology/vstatlas/surveymap.jpg (or literally any similar plot, if we imagine borders represent individual FITS).
Of course ds9 can do this, but I have many files and it is awkward to do that way. I check other questions suggest use of Mosaic - can't find proper example, also try using Montage and both its python wrappers without success. Also browse astropy examples without any luck.


texts: We know that the estimated time until a binary system merges is given by: $$\dfrac{5c^5r^4}{256G^3(m_1m_2)(m_1+m_2)}$$ But what is the formula to determine rate of decay, in some sort of distance unit, per unit of time? Also, what are terms are $r$ and $m_i$ written in?


texts: How many stars and galaxies can be seen by the naked eye?

texts:
How many of the luminous dots that we see naked are galaxies and not stars from our galaxy?

I imagine that the majority of the luminous points that we see naked eye during the night, are actually stars from our galaxy. But how many of them are other objects (other galaxies, nebula, etc.), excluding planets from our Solar System?


texts: Items at the Event Horizon vs. colliding Black Holes

texts: Items reaching a Black Hole's Event Horizon seem to be frozen in time from the point of view of an outside observer. On the other hand, Black Holes collide and merge, in finite steps, producing gravitational waves observable to outside observers. How is it possible these two facts not to contradict each other?


texts:
What is the planet relative to earth that shows the greatest change in apparent brightness?

I think the answer is Mars, and a quick google search reveals it is Mars.

I have my reasoning here, but I am not sure if I am correct.

One way to do this problem is to get all the planets and collect their perihelion and aphelion distances from the sun, and see the different.

However I want to be able to explain this without the extensive use of numbers.

Here is my explanation:

The closest planet to us is Mars, both in terms of its aphelion and its perihelion distances

The Earth-Mars distance varies between 2.66 AU and .38 AU

Mercury is almost always blinded by the sun, so we rarely see it in the sky

Venus is frequently viewed as a crescent, and thus its apparent brightness also dimishes

Jupiter and rest of the superior planets are simply too far away for us to notice a big differene

Are all my points correct here?


texts: Is there any habitable exoplanet around Tau Ceti?

texts: I was wondering.. is there any habitable exoplanet around Tau Ceti?


texts:
Edit: Geometric answer from Math.stackexchange

I am trying to understand where the circumpolar equation comes from.

$\delta \geq \frac{\pi}{2} - l$

I can understand that the sort of "rotation" of the reference frame that one experiences in horizon coordinates. I'm not sure where to go from the fact that a star's lowest altitude must be greater than 0.

It is pretty simple to show that $a = l$ and $c = \frac{\pi}{2} - l$. $a$ in this case is the altitude of the north celestial pole. What I need to do is show that the angle of the cone made by a circumpolar star with $\delta + l \geq \frac{\pi}{2}$.

You can deduce that $a_{min} \geq 0$ for a star to be considered circumpolar. I just need to do this geometrically in terms of its declination from the celestial equator.

Edit: I think my latest attempt has found a solution.

For a star to be circumpolar, the star must, at the very least, come above the horizon.

$0 \leq a \leq \frac{\pi}{2}$

In the celestial sphere coordinate system, $c$ can be found to be

$c = \frac{\pi}{2} - \delta$

In the horizon coordinate system, $c$ can be found to be

$c = l - a_{min}$

Therefore

$l - a_{min} = \frac{\pi}{2} - \delta$
$\\$
$l + \delta = \frac{\pi}{2} + a_{min}$

$a_{min}$ can be set to 0 because it is the lower bound for circumpolarity.

$l + \delta = \frac{\pi}{2}$

$\delta$ must be in the cone made by the star's rotation about its declination angle. Therefore

$\frac{\pi}{2} - l \leq \delta$


texts:
Has there been any study to suggest that either quasars of gamma ray bursts are more detectable from further distances in the universe?

I've read that quasars are the most powerful energy in the universe but gamma ray bursts are detectable from further distances. Logically one would think that if some entity's energy were detectable from further distances than another entity's energy, then it'd be the entity with more energy than the other.

I'm just looking to see if anyone has any definitive study references that suggest why either quasar or gamma ray bursts have more energy than the other and to explain why if true the one with less energy is visible from further distances than the one with more energy.

I'm not really an astrophysicist type guy but I do some reading and watch different scientific shows that interest my since a lot is still unknown, so I'm hopeful someone here more familiar with this can answer or clarify this for me or give me some references to anything that gives clue to any of this.

I apologize for not having direct reference to any readings I've read that bring about this question in my mind but I know I read something that said gamma ray burst are detectable from further distances than any other energy entity and I've also read quasars are the most powerful energy sources in the universe, so I'm just looking for some clarify on this topic whether right or wrong with anything further you can provide.


texts: How to calculate the redshift of a line in a bunch of line spectra?

texts: If in a spectra, there are many lines, mixed with gravitational redshift and non-redshift lines. For example, in this paper, they identify lines both redshifted and no-redshifted. Then how can i distinguish those redshifted from those do not?


texts: Calculating angular diameter amplitude from angular diameter variations of Cepheid variable

texts:
I'm a high school student but have as much interests in Cepheid variable as the professionalists. I'm trying to derive independent distances to Cepheid with the Baade-Wesselink method.

I got the angular diameter variations data from the paper Cepheid distances from infrared long-baseline interferometry but I'm stuck from the afterward. I want to know how equation 9 in that paper is derived. Where did the number 9.305 suddenly came up?

The equation is the following:

θLimb-darkened model angular diameter = amplitude of limb-darkened observed angular diameter + 9.305 (∆D/d)

It seems like I need that equation to derive the distance from the angular diameter values with radial velocity values.


texts: What would be the impact on the formation of a new star (protostar) when a nearby star (within 10 LY) goes supernova? Will the force of the explosion (once it arrives in the Molecular Cloud where the star is forming) disrupt the process to the point where the new star cannot complete the 10-million-year process to form a solar-type star?


texts: Need some sources on radiotelescope sensitivity

texts: Could anyone direct me to papers with info on minimal flux that can be registered with radiotelescopes?


texts: I am leading a project for my High School NASA Club, and am looking for some advice. The goal of our project is to use a Raspberry Pi to control a camera that takes photos of the sky, and uses the photo to calculate sky glow, and create a map of where it in the sky is there the most sky glow. After calculating sky glow the device would add it to a plot that shows how sky glow varies throughout the year, and creates a nightly twitter post rating the night for Astronomy. I currently have working code that when given a picture, it will convert down to greyscale, and then find the brightness map, and find an average sky glow. The part that I am stuck on is in figuring out what kind of camera to use. I don't know if the RPi camera module would work for taking photos of the stars, or if I would be better suited using a USB webcam with it. I am not sure what factors are important in this decision at all. Also, once the camera is set up, what would be the best process for calibrating it to get values that agree most closely with accepted ones? I couldn't decide if this should go on the Astronomy, Raspberry Pi, or Photography Stack Exchange, but I chose this one since sky glow may be something astronomers calculate a lot. Thank you for your help.


texts: Why do we keep orbiting through the Perseid meteors?

texts: I understand that the meteors originated from the the comet Swift–Tuttle. Presumably they left the comet with some velocity relative to the solar system. As comets follow elliptical orbits presumably the meteors are generally also following an elliptical orbit. And as the earth follows a circular orbit, why do we keep meeting them every year despite being on different orbits?


texts:
The usual explanation is that escape velocity of a black hole is above speed of light, therefore nothing can escape because nothing can move faster than light.

This is however a fallacy, because escape velocity only pertains to bodies "without propulsion", or in other words without any force other than gravity acting on them. As an example I could theoretically escape Earth's gravity at a walking speed, just give me a staircase and I could walk all the way to the Moon. No need to accelerate to 11 km/s at any point.

How does this affect black holes? Why cannot I have a staircase and climb out?

I know the staircase idea must be wrong, but I would like to know why. My guess is that I am applying Newtonian mechanics to a problem which requires general relativity approach, because Newtonian mechanics approximation fails badly around such massive gravity fields?

Anyway, according to Newtonian mechanics one can easily escape a black hole, even if we assume that speed of light cannot be exceeded.


texts: Would an analogue of the definition for planets also work for moons?

texts:
This is a follow up question to What exactly is a "moon"? The conclusions I draw from James K's answer is that the IAU should define what a moon is. They haven't done so yet, but they should.

The obvious way to go about defining "moon" would be to build on the definition of a planet:

Planets must be in orbit around the Sun
Planets must be round due to their own gravity
Planets must have "cleared the neighbourhood"

This immediately suggests the equivalent definition of a moon:

Moons must be in orbit around the host planet
Moons must have [minimum size]. Unfortunately being round doesn't work since not all moons are round (c.f. Mars' moons, unless one reclassifies those as not moons)
Moons must have "cleared the neighbourhood" of their own orbit, i.e. in their particular orbit around the host they are by far the most massive body

Would such a definition work? If so, why hasn't the IAU also defined moons? It seems so natural after all. If not, what's the catch?


texts: Can Mercury hit Earth or Mars in the next 5 billion years?

texts:
Mercury is the most unstable planet of the solar system, with an oscillating orbital eccentricity (between 0 to 0.45). It seems there is a 1% probability in the next 5 billion years Jupiter and Mercury might enter in a 1:1 orbital resonance.

The ending variants for this event are :

colliding with Venus
colliding with the Sun
being ejected from solar system.

Can it hit other targets on its way out of the solar system beside those enumerated, like Earth, Mars or the outer planets?


texts: How can I calculate moon phase and height?

texts:
How could I know what phase the moon is in when it's high in the sky during sunrise?

How could I tell in general what its height in the sky will be at any given time?


texts: What earth organisms would survive if they arrived on Mars?

texts:
Contaminating other planets is a concern in space exploration. My question is what organisms, or small collections of organisms, from earth would be able form a self-sustaining population on Mars? I'm not worried about how they might get there, so deep sea microbes and the like which would have to be purposely transported are in scope.

This is not the same as this earlier question, as that one does not consider sustainability.


texts: How would Special relativity explain past-present-future if thinking about 13-billion-light-years distanced objects

texts:
Halo.

I'm interested in how would Special relativity comment on this:

Suppose there are 2 objects, A and B. They are 13 billion light years away from each other.

When the light from the B object reaches the A, the observer at the A object sees what the (suppose planet, people, objects) were like 13 billion years ago on that object. Let's say it is also an Earth-like planet where the living creatures like exist.

And if the observer at the A object would like to imagine, what is now happening there, what would be the result:

Surely, he would have to wait another 13 billion years to actually
see
Would that mean that the inhabitants (if any) or just matter and
objects at object B exist in future relative to A ones. And vice
versa? If yes, then they both live in present, past and future
tenses at the same time?

Or have I completely lost my way of properly understanding the Special relativity?


texts: How to find the Orbital Period of two Binary Planets around a Single Star?

texts:
This is kind of a weird one. Bare with me! :)

Background: (Skip to the math below if you just want to answer questions)

In my favorite game, Elite: Dangerous, I "own", or occupy I guess, a system named "59 Virginis", based on the real life counterpart inside the Virgo Constellation.

The game itself houses a unique, very close to life-like system called "Stellar Forge", which uses as close to real life accuracy with physics as possible.

In this system, my own group's "Home" is the Earth-Like planet "59 Virginis 4", which is in a binary (tidally locked) orbit with 59 Virginis 3 (a class III gas giant).

I am trying to create an "Alien Calendar" of sorts, and a key component of calendars is the year, or how long it takes to rotate the sun once. This is semi-important, seeing as the home planet itself is titled on it's axis, meaning the Earth-Like will have seasons! Meaning having a yearly cycle of seasons would be awesome, and making a calendar would be lots of fun!

This is where I'm incredibly stuck. The game doesn't outright tell us how long it takes for these two objects to circle the sun, but it does give a lot of extras on possibilities of finding it!

I've tried a few different methods, but I don't think I know enough math to be able to find such a thing.

Here is the information we have currently:

59 Virginis 3 (the Gas Giant)

"distanceToArrival": 964 light-seconds, (this value changes based on where the planets are in binary orbit at the time of information update),
"earthMasses": 2027.9021,
"radius": 72755.136 km,
"orbitalPeriod": 80.53994791666666 d,
"semiMajorAxis":  5.898593314670755e-5 au,
"orbitalEccentricity": 0.114867,
"orbitalInclination": 0.071319°,
"argOfPeriapsis": 246.659607°,
"rotationalPeriod": 1.7743948929398148 d,
"rotationalPeriodTidallyLocked": false,
"axialTilt": -1.236412°

59 Virginis 4

"distanceToArrival": 989 light-seconds,
"earthMasses": 1.795525,
"radius": 7228.5435 km,
"orbitalPeriod":80.53994791666666 d,
"semiMajorAxis": 0.0666149271869282 au,
"orbitalEccentricity":0.114867,
"orbitalInclination":0.071319°,
"argOfPeriapsis":66.659615°,
"rotationalPeriod":80.57560763888888 d,
"rotationalPeriodTidallyLocked":false, (it's pretty damn close tho)
"axialTilt":-0.293082°,

The above information can be taken from here which is the most accurate version that can be given. If you want it in an easier to read format, click on the bodies 3 and 4 here

I know that the orbital period of a binary object is this:

$T= 2\pi\sqrt{\frac{a^3}{G \left(M_1 + M_2\right)}}$

Which only describes the orbital period of the two objects around eachother, right?

Ideas that have been thrown around:

Using Kepler's Third law, which states "The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit."
Using the two object's "distance to arrival" to find the distance between the two objects, then finding where on that line the center of mass is, and then using that center of mass point to find the distance to the star again from that point, then using that distance to find the orbit. (This sounds like the most reasonable from my point of view, but possibly the hardest)

Here is a really bad paint drawing of what I think the one above could look like:

Possibly finding the sidereal period somehow, or synodic period in relation to the giant and the sun?

If you've gotten this far, you've already helped me more than most! Any advice in relation of where to go next would me MUCH appreciated! Thank you!!

The next step after finding how long it's sun's orbital period is to find how many years to reset the whole cycle, kind of like a leap year, but with the binary orbits. That should be slightly easier, but I'm stuck on this for now! :)


texts: How to install Distro Astro operating system?

texts:
I've recently started a project on Galaxy spectroscopy for which I had to use IRAF software. During the search for IRAF software, I came across a platform which said that Distro Astro is a very good OS Amateur astronomer should check out. I have searched the internet for the procedure and am an amateur in OS installation and other stuff.

Does anybody know the detailed procedure of installing the software?


texts: Why do some solar eclipses' umbra cross arctic/antarctic regions?

texts:
The ecliptic (Earth's orbital plane) is inclined 23.4 degrees. This is the same as Earth's axial tilt. The Moon's orbit is inclined 5.145 degrees to the ecliptic.

Therefore, the maximum latitude where a solar eclipse produces an umbra should be 28.545 N and 28.545 S.

But this is clearly not the case. Wikipedia has a huge list of solar eclipses with charts. Here's one that goes very near the south pole. How is this possible?

Edit: Actually I missed something. Since the Moon and Sun must be in the same place, and the Sun can never exceed 23.4 degrees declination, solar eclipses must be bound by 23.4 N/S. I should not have added the two angles. Thanks to user:berrycenter for pointing it out.


texts: To study the physics of black holes (What we presume as really dense matter) could we not simply simulate that here on earth? What if we took 1 hydrogen molecule and just crushed it with enough force that I'd fall apart and then crush it even more? I'm curious what would happen.


texts: Does the universe expand at the same rate everywhere in the universe?

texts: Specifically, I am wondering if some areas of the universe expand faster than other areas and whether the faster expanding areas diffuse the expansion through the slower expanding areas or does the expansion occur at a uniform rate throughout the entire universe.


texts: Is it possible for redshift to be caused by light being somehow slowed down during travel?

texts: In other words, is the assumption, that redshift is caused by universe expansion, definitive? Or are other theories still being considered? Could it be, that light is being slowed down by dark energy, dark matter, massive objects, etc., as it travels through space?


texts: Do stars tend to fuse all hydrogen even when the mass is small?

texts:
Suppose a brown dwarf requires only one more hydrogen atom to become massive enough to start fusion, what will happen if we add a hydrogen atom to it?

It fuses until the mass is dropped and then fusion stops suddenly
It starts to fuse until all hydrogen use up

Which one is correct?


texts: WOW! signal calculation of point source intensity

texts: The WOW! signal's measurement took the average signal strength over 12-second intervals. The average intensity's peak was just over 30-1 signal to noise, meaning that the 12 second interval had an average strength of 30-1. But evidence suggests the signal came from a point source. Has anyone ever done the math to estimate the point source strength? Is this even possible?


texts: I was wondering, is the surface of Venus so hot that it would glow red in the dark (e.g. on the dark side of Venus)?  I am working on making a solar system simulator, and that would make for a great level if it is accurate.


texts: Could there be liquid water on Uranus? Are there any indications that there might be?

texts:
Like most planets, Uranus has a very cold outer atmosphere and a very hot core.  What we see is a very thick primary atmosphere with plenty of hydrogen.  Deeper in, we might suppose that water condenses in clouds at maybe 200 atm of pressure. (Above that it would be ice crystals)
By "might suppose", I've used the dodgiest source conceivable to plot the atmospheric conditions on a phase diagram of water, shamelessly thefted from Duke here, whereby I mean I took a Wikipedia contributor's dotted blue line, "an extrapolation of the 1987 Lindal et al. data (solid blue) using a constant lapse rate of 0.82 K/km."

Given this impeccable data (or other data you might contribute!), it appears that the extrapolated atmospheric conditions of Uranus take a shot straight down the fairway toward a layer of liquid water.  IF it's there.  The usual treatment of the topic says that there is an atmosphere over a layer of ices ... without many specifics, since after all nobody can see it.
But do we have any indication at all?  Is there a reason to say probably yes or probably no to the question of whether a probe dropped into Uranus would reach 374 C (the critical temperature of water) before it would reach a liquid (probably, in that case) surface of water?
(Note: I recall seeing some simulation of the overall temperature issue before, but with a range of outcomes, and this question is prompted by this report that MgO dissolves extensively into H2O in Uranus, potentially altering the internal heat distribution.


texts: Why are there uneven bright areas in this photo of black hole?

texts: In the recently released photo of a black hole shown above, which was created by using data from EHT, why is the lower region brighter than the one above? Is it because of the rotation of the accretion disk? Also what is the orientation of the accretion disk? Are we looking at it head on?


texts: If two binary brown dwarfs began to exchange gas with each other, would they both ignite into stars?

texts: If two brown dwarfs got close enough to exchange gas with each other, could it trigger nuclear fusion within their cores? Is it possible for two brown dwarfs to orbit so closely and become a rocheworld in the first place?


texts: Are there more images with different filters available for the April 6 Hubble image of Jupiter?

texts:
This recent HST image of Jupiter near opposition is described here and here and was made under HST proposal 14756. It was taken with WFC3/UVIS. It is made from three images using narrow-band filters at 395, 502, and 631 nanometers.

I am wondering if there were additional images recorded with filters at other wavelengths. If so, how could I go about finding them?


texts: I want to start observing the night sky, what advice would you give to get started?


texts: Is there an IRAM satellite that measures thermal radiation at 250 GHz, or was this a ground-based instrument?

texts:
The Nature Research Letter A Pluto-like radius and a high albedo for the dwarf planet Eris from an occultation (also here and here) says about half-way through:

We now reassess Eris’ surface temperature in the light of our new results. Measurements by the Spitzer22 and IRAM11 satellites imply disk-averaged brightness temperatures of Tb=30±1.2 K and Tb=38±7.5 K at 70 and 1,200 mm, respectively.

11Bertoldi,F.F.Altenhoff,W.Weiss,A.,Menten,K.M.&Thum,C.Thetrans-neptunian object UB313 is larger than Pluto. Nature 439, 563–564 (2006).

22Stansberry, J. et al. in The Solar System beyond Neptune (eds Barucci, M. A., Boehnhardt, H., Cruikshank, D. P. & Morbidelli, A.) 161–179 (Univ. Arizona Press, 2008).

The caption of Figure 1 of reference 11 (Bertoldi et al. 2006) mentions:

The 117-element MAMBO-2 camera has an effective frequency for thermal radiation of 250 GHz, a half-power bandwidth of 80 GHz (210–290 GHz), and a beam size of 10.7 arcsec (corresponding to 760,000 km at a distance of 96 AU).

and I think it refers to

Welcome to the home of MAMBO; the "Max-Planck-Millimeter-Bolometer"
IRAM/MAMBO2.250GHz

Question: Is the use of the word satellites in "Measurements by the Spitzer22 and IRAM11 satellites..." correct, or does the latter refer to terrestrial measurments?


texts: What are possible methods of discerning a starburst AGN composite galaxy?

texts:
I'm currently working on an independent project that involves discerning starburst galaxies that are themselves within active galaxies.

I would assume radio observations would be best to discern star formation rates, and X-ray to understand the AGN in greater detail. However, assuming we have only optical data could I use the relative strength of the OII flux to the OIII flux to better understand the star formation rate of an active galaxy? Spectroscopic surveys of distant galaxies, particularly for redshifts between $z$ ≈ $0.4$ to $1$, routinely use [O II] $λ3727$, a prominent nebular emission line in H II regions, to estimate SFRs. If (high-ionisation) AGNs experience substantial levels of ongoing star formation, the integrated contribution from H II regions will boost the strength of the [O II] line (compared to, say, [O III] $λ5007$, which can be largely ascribed to the AGN itself).


texts: how do I read grades for knowing where I have to look for watching the ISS?

texts: I want to watch the ISS with naked eye using this tool NASA gives https://spotthestation.nasa.gov/sightings/view.cfm?country=Argentina&region=None&city=Santa_Fe#.WCRzKS3hC1s but I dont know how to read grades in the sense of which direction I have to look to, in order to see the ISS.


texts:
In the GR Einstein's interpretation was in terms of a deformation of the geometry of the space ending in a curvature of spacetime.

I've thought that mass could do this by interacting with other mass and causes gravitational waves which makes the space curved. But a prof told me that this isn't the case. For example the gravitational waves caused between the earth and the sun is just a very tiny effect which causes spiraling earth to the sun which is of little effect.

So mass is establishing the geometry of the space and the time. But the question remains how is mass doing that (curving spacetime)?


texts: How can we avoid needing a leap year/second?

texts: Given the Earth's current speed around the sun and current rate & axis of rotation, what is the best way to keep time to avoid a leap year? How many hours should we have in the day and days in a year would keep things balanced to not need to add or remove days from the year? Further, how many minutes per hour and seconds per minute should we have to avoid a leap second?


texts: Does the lower mass of Uranus suggest it swapped position with Neptune?

texts: As Uranus is less massive than Neptune, would this be evidence that it was formed further away from the sun than Neptune, and later it swaps orbit with Neptune to move closer to sun?


texts: Help in determining the features of an unusual, fictional star system

texts:
(Hope this is the right Stack-exchange site for this question)
I'm working on a sci-fi RPG campaign, set on a very atypical location. Since this is a work of fiction, there's enough room for speculation and artistic license. But I'd still like to start from a more astronomically sound perspective.

Background
Consider the following system:

A single moon, with properties roughly comparable to Earth's so that it can support human life (1G gravity, Nitrogen-Oxygen-Carbon atmosphere, soil and liquid water etc.). This moon is tidally locked with it's planet.

A single gas-giant planet, with a size roughly comparable to Saturn. Orbiting it's star so that it completes a cycle about every 12 earth years.

A Star, larger than Sol and emitting enough energy to keep the moon in a comfortably warm temperature.

The features above may be altered to ensure possibility of human survival on the moon, and if possible, to keep the moon tidally locked with a large planet.
Now, the complications (inspired by a short story by Larry Niven):

The whole system (star, planet and moon) are traveling at a relativistic speed across the galaxy.
(Based on the theory that some parts of the universe might be made from anti-matter) The whole system is made from anti-matter, but is currently traveling through a normal-matter area of space.
Note: the "human life" on the moon are also based on anti-matter...

Finally, the story will begin with a small community of primitive "anti-matter humans", living on the moon in a place from which the planet fully visible, and at the zenith.
The Questions
I'd appreciate ideas, insights and comments on the following:

From the moon, how will the sky look like?
What will the equivalent of a "day and night cycle" (sun-up to sun-up) look like?
Will the winds on the moon have any "non-earthlike" patterns?
Considering that at relativistic speeds, the odds of the bodies in the system hitting hydrogen particles in the vacuum of interstellar space are significant (is that even correct?), will that create any special aura or energy streams from the sun or planet?

Thank you for your help.


texts: I am reading a book on spherical astronomy. Surprisingly, my problem is not with the mathematics. The author mentions several times words like 'eastward movement' and 'westward movement'. For example, the Earth revolves around the Sun in an eastward direction. When I look at the diagram from the perspective of someone above the north pole of the Sun, the Earth seems to go around the Sun in an anticlockwise manner. My problem is I don't understand what is meant by eastward movement ? Can someone explain what this means without ambiguity ? Is there some sort of a right-hand rule that one can follow such as Fleming's right-hand rule in electricity ?


texts: Can protoplanetary disks form main-sequence stars?

texts:
As has been pointed out by @Envite in the context of a more general discussion (see Generalised planets?), there seems to be a moderate possibility for protoplanetary discs forming main-sequence stars (MS stars).

Main argument here is that Jupiter has mass of about $10^{-3}M_\odot$, while in principle for an object to be a MS star, it would need the mass of at least about $0.07M_\odot$. Also, there is a handful of systems, such as HD 29587, which contain a star and a brown dwarf, possibly forming from a protoplanetary disk.

Hence, a few questions. Can low-mass MS stars form in protoplanetary discs? If no, why? If yes, how often does it happen? If yes, which implications would it have for the subsequent dynamics of the planetary system, if one of the objects became a MS star, as opposed to it being a brown dwarf?

Again, thanks to @Envite for the idea.


texts: Simulation packages or theory to work with gravitational collapse of massive molecular clouds?

texts: So, I am getting interested in gravitational collapse of large molecular clouds that fragment and collapse into multiple stars. Are there well-known popular academic simulation packages that simulate these, for research purposes? Or are there papers or references I can use to write my own?


texts:
Say there's a young s-type binary star system with 2 Sun-like stars. From what I understand for a stable orbit to be possible, a planet's farthest orbital point from its host star has to be around 1/5 or less of the total distance to the companion star (source: Wikipedia), but I'm unsure if this works for a gas giant.

The gases it would use to form would feel some effect, right? What's the closet the companion star could be to the host star while still allowing for a gas giant to form and have a stable orbit?

Make any assumptions you have to for this question and thanks for any input.


texts: Existence of wormholes and exotic matter

texts: Is it really possible for bending of space time to travel to another distant place in universe. Wormhole has two mouths which one is a black hole and another is a white hole which could let us out. Which is commonly called as Einstein-Rosen bridge, is there any pictorial proof of wormholes other than the theoretical proof by Einstein. Is there a possibility of having exotic matter which helps the wormhole to keep open up always??


texts:
So I've been having fun with trying to determine where the perihelia of different planets are (using JPL Horizons).
And by the time I got to Saturn, something weird started happening. I've noticed that at some moments true anomaly (that I've used to track perihelion time) started falling rather than rising. I've also noticed that it seems to be only a slight effect, which also demonstrated ~ 17 days period. So I though: perhaps that's the effect of Titan, right? Problem solved.
Yet now I look at Neptune's true anomaly (see the last column on the picture) and have absolutely no idea what's going on.
Can somebody explain, please? How is something like that even possible? It's like the planet decided to go backwards for a change.
My settings are:
Ephemeris Type [change] :   OBSERVER
Target Body [change] :  Neptune [899]
Observer Location [change] :    Sun (body center) [500@10]
Time Span [change] :    Start=1900-01-01, Stop=2100-12-01, Step=1 Y
Table Settings [change] :   QUANTITIES=18,41
Display/Output [change] :   default (formatted HTML)


texts: Will it be dangerous when close stars become Red Giants?

texts:
What will happen if close stars like Sirius A or Procyon_A become red giants? Is it dangerous for the Earth and life on it?

If for Sirius A we still have 500-800 million years, but Procyon A might become a red giant in 10 to 100 million years, by astronomical scale it is very soon.

Or nothing will happen with the Earth, because it is still too far?


texts: In how far is Hipparcos data still being used?

texts:
I remember being excited when ESA launched Hipparcos in 1989 and was in operation until 15 August, 1993. Later, Gaia came into operation in 2013.
Reading What is the overlap between the Gaia and the Hipparcos catalog? I started wondering whether any of the Hipparcos data is still being used and for what. Could somebody please guide me with a concise explanation?
Reference

https://www.cosmos.esa.int/web/hipparcos/catalogues
https://www.cosmos.esa.int/web/gaia/data


texts:
I was watching Venus with the naked eye yesterday at about 7 pm and I noticed that it was flickering, almost like a star.

I have always learned that planets don't flicker to the naked eye, only stars (indeed, every other time I've seen a planet it wasn't flickering), so I was rather confused. I even checked Stellarium to make sure that what I was seeing was, in fact, Venus.

Does anybody have an explanation for that behaviour?

Thank you all.


texts: How do we know that other planetary systems aren't composed of antimatter? If they never touch matter, then they don't annihilate. All their observable physics (spectra) are the same as matter, right?


texts: Do stars of a galaxy change their positions relatively to each other?

texts:
Complete astronomy noob over here who would be happy if he get a simple answer (and who is also aware that this may be not possible)...

I've learned from a tv documentary that the stars at the edge of the galaxy are not traveling more slowely than the ones closer to the center.

Does this also mean that all stars in a galaxy do not change their positions relatively to each other?

To put it simply: If I note the relative position of a star compared to our sun, and then do it again 10 (100 / 1000) years later: Are the coordinates of that star identical or will the star be in a different relative position?

I would assume that the star is in a completely different (relative) place given the speed star systems are travelling because the galaxy is rotating AND the "fact" (read: I don't know if this is really a fact) that the solar system is oscillating through the galactic plane.


texts: Intuitive explanation for why the Doppler effect (and red-shift) happens?

texts:
I was looking for an intuitive explanation as to why the Doppler effect happens. I haven't found any, but this is what I thought:
-Waves emitted travel at a constant speed
-The source emits a wave
-If the source remained still, then in a given time, there would a large distance between the emitted wavefront and the stationary source
-But if the source starts moving, the speed of the source relative to the wavefront is higher.
-This means in a given time, the distance between the emitted wavefront and the source will be lower
-So when the source (while moving) emits a new wavefront, the distance between wavefronts will also be lower
Is this explanation wrong? It suggests that the Doppler effect with light will be barely noticeable, because even if a source starts moving, because the speed of light is so high, the source speed will still be small relatively, so there will be a small (basically insignificant) decrease in distance between wavefronts. So how can red-shift be so noticeable, unless recession speed is extremely high (so high that it is actually significant relative to the speed of light)?


texts: Which part of the milky way does the image show?

texts:
As a general question and more in specific regarding the image above: Which part of the milky way, planets, stars, clusters are included in the image?

How can I know which entities are included in different pictures?


texts: How can we learn the inclination of the orbiting plane of the exoplanets using radial speed method?

texts:
I heard about the radial speed method to discover exoplanets a few years ago, however, these questions kept confusing me. If we know a planet star of mess $m$ is orbiting a star by interpreting the  radial speed of the central mass, how can we tell wether this is a even more massive object whose orbiting plane has a larger inclination? Further more, even if we know the angle of our sight and the axis of orbit, can we determine the direction of orbit?

Thanks in advance.


texts: How to improve the quality of an image of the planets when taken through the eyepiece of a telescope with a dlsr camera

texts: I have a reflector telescope with a 114mm aperture and 1000mm focal length.  I find that to get the clearest image I can use a 4mm eyepiece, or with my 5x barlow a 20mm however, seeing that I am very interested in astro photography when I try to photograph Jupiter for instance, through the telescope eyepiece, being the size of about a green pea seen through the scope, there is not enough pixel data for my canon eos t2i rebel dlsr to capture a clear photo. Is there some way to enlarge the photo for the camera without blurring the planet's details from over magnification?


texts: I am trying to find the position angle of a star whose Ra and dec are known. I have already looked at the 'Measuring misalignment between two positions on sky' question and answer, but the answer gives an undefined solution if one of the points is the North Celestial Pole (NCP). I would appreciate if anyone could tell me how to find an object's position angle given its equatorial coordinates.


texts: Is it certain that dark matter consists of particles? (And not just curved space)

texts:
The main idea about dark matter today seems to be supersymmetry. That there exists a set of yet undiscovered fundamental particles which (in a mathematically most convenient way) mirror the particles already known.

But has it been established that dark matter consists of particles at all? Couldn't it for example be a continuous property of space itself in some regions? That some parts of space are curved without the influence of any particles or waves. Is anything but particles ruled out?


texts: What is the difference between spectroscopy, spectrography and spectrometry?

texts:
Spectroscopy is the study of spectra, spectrography is the writing of the spectra, and spectrometry is the measure of spectra. So from an etymological perspective, there is no real difference between the three. On arXiv, there is for instance an article that uses the term "spectrometry" in the title, but "spectroscopy" in the abstract (several, actually) to designate what seems to be the same thing.

But perhaps are there differences between the use of these terms in astrophysics?

There are certain techniques that are designated with one term rather than the other (e.g I haven't seen any mass spectroscopy, but have seen mass spectrometry), but is there a conceptual difference between spectroscopy and spectrometry?


texts: How can a star be "thrown out of the Milky Way" by Sagittarius A*?

texts:
A recent publication reports that a hypervelocity star on course to leave the galaxy was found, likely accelerated by the black hole at the center of the Milky Way in a process predicted by Jack G. Hills in 1988. A sentence from that abstract insightfully points out:

  The discovery of even one such hyper-velocity star coming from the Galactic centre would be nearly definitive evidence for a massive black hole.

The process involves a binary system coming close enough to the black hole that one of the stars is "captured" and the other one is flung out.

As far as I can see, the escaping star can at most end up with the kinetic energy of the other star on top of its own, accelerating it by a factor of $\sqrt{2}$ if the stars have the same mass. A "slingshot effect"  is impossible because the central black hole is stationary relative to the galaxy.

Does that mean that this effect necessarily involves binary systems with very different masses so that more energy is available for transfer to the lighter star? Or are there other effects at play?


texts: What symbol is used for the moon when doing calculations?

texts:
When doing calculations the mass and radius of the Earth can be represented by $M_{\oplus}$ and $R_{\oplus}$ respectively.

The planets all have their own symbols which can easily be found online.

But what about the Moon ?

Usually something like $M_☽$ is given but is that standard ? Are there other versions of expressing the moon ?


texts: Using Tully Fisher to measure Distance Problem

texts:
Can anyone explain by looking at the solution in the pic, how did the cos (i) came about?
I guess they are getting this from the major-minor axis info in the question, but I am not sure about the derivation.

Also v=300/sin(i), what is this formula?

Many thanks :)


texts: Pulsars: How do astronomers measure minute changes in period (~picoseconds per year)?

texts:
I've been to some talks that mention how stable the period of a millisecond pulsar is over long periods of time. Recently, it was mentioned that astronomers have calculated the change in period over time to be less than 10^-12 seconds per year for several pulsars. No one I've talked to seems to know any details of this calculation.
How do we calculate such small differences in period? How much data must be collected and what are the exposure times for imaging such rapid phenomena?
A source/paper would be excellent. I apologize that I don't have a citation for the 10^-12s figure, but the lack of citation is mostly my reason for posting this question.


texts:
On Earth, the sky is blue.

What color would the sky be on other planets in our solar system? What about outside the solar system? Are there planets with purple skies, or green skies? Or are the all just blue?


texts:
Most of the known gas planets (Jupiter, Saturn, etc.) are huge. They are even called "gas giants".

is it possible to exist a gas planet around the size of Earth? If yes, why; if no, why?


texts: Which eyepiece should I use to view planets with a Slokey Skyways 50080 telescope?

texts:
I am new to Astronomy. My husband bought a Skyways 50080 for me for my birthday. It's an f=500mm refractor made by Slokey. The 80mm aperture makes it a fast f/6.25.
It came with three eyepieces; 25, 10 and 6 mm, and a 3x Barlow lens.
I can see the moon very clearly, however I can't see Mars very well. I use the 10 mm eyepiece when viewing the planets, I am not sure if this is too high a magnification or not.
Which eyepieces should I be using to see the planets with this type of telescope? Any suggestions will be greatly appreciated.


texts: Can BY Draconis variable stars sustain any form of complex life?

texts:
I wanted to ask this, because while I was working on two fictional planets back on the Worldbuilding SE, one of them being habitable, which orbit the 61 Cygni A star of the 61 Cygni binary star system, I realised something: 61 Cygni A is a BY Draconis variable star.

Here is the Wikipedia article link for more information.

My problem is: can a BY Draconis variable K-class star like 61 Cygni A permit the development of complex life, unlike its partner, 61 Cygni B, is a flare star that can barely get life past the primordial stage, if any planets are there? Or does it have some problems in that regard? If so, what are those problems that a habitable world in that star system could have, and other star systems of the BY Draconis variable star category?


texts: How does dark matter interact with black holes?

texts: Is dark matter trapped near / in black holes, like regular matter is, or does something different happen with dark matter?


texts: How to calculate galaxy bolometric luminosity?

texts:
I am a bit confused by bolometric corrections. If I have an x-ray luminosity in the 2-10 keV band, how does one convert that to $L_{bol}$?
From Netzer's book The Physics and Evolution of Active Galactic Nuclei I got these bolometric correction factors:
Optical: $BC_{5100} = 53 - log(L_{5100})$
and x-ray: $log(L_{5100}) = 1.4\times log(L_X) - 16.8$
where the bolometric correction for the x-ray luminosity ($L_X$) is obtained in two steps, using the equation for the optical BC again. The index $5100$ stands for the optical continuum measured at $5100$ angstrom.
I can't figure out what I have to do with this $BC_{5100}$ once I've got it. Multiply by $L_X$? The book says "(...) BCs, that can be used to convert a single-band measurement of $L$ into an approximate $L_{bol}$."
I'm happy to use correction factors defined elsewhere instead of the ones I quoted. I just want to calculate an estimate for $L_{bol}$ for my galaxies.


texts: Short term changes in celestial coordinates of stars

texts:
The right ascension and declination of stars change on a day to day basis. Some of the changes are quite large. The right ascension of Polaris, for example, changes by more than a degree over the course of some years. The excursions are irregular. Their time and extent differ from year to year. I know that precession will change coordinates of all the stars over tens of thousands of years, but I can't find an explanation for these short term changes.

Polaris started 2017 with a sidereal hour angle of 316 deg 23.9 min. The SHA increased monotonically to 317  3.6 by the end of April, then decreased in small steps to 315  58.3 by November 9, then increased again, ending the year at 316
 6.2.